A self-oscillating transistor switch circuit of this general type is disclosed in EP0334886B1. The bridge switch transistors are controlled by a control circuit so that each switch transistor is switched “off” when certain conditions are met and are switched “on” only when the current in the bridge is very low or essentially zero. Thus, the power dissipation during the switch “on” operation is very low. Further improvements of the basic switch circuit are disclosed in WO02080342A1, WO02080343A1, and WO03001656A1.
The control circuit is constructed so that the switch frequency increases with decreasing load. Thus, the power dissipation in the switch transistors may increase with frequency, which may result in that the power dissipation at low load may be excessively high.
The switch circuit may be provided with transistors of the MOSFET type (metal oxide semiconductor field effect transistor), which can operate at high frequencies up to and above 500 kHz. In certain applications it would be desirable to use bipolar transistors, such as transistors of the IGBT (insulated gate bipolar transistor) type. However, the power dissipation in an IGBT during switch off is larger compared to a MOSFET, partly because of a tail current that may last for several microseconds. Thus, such IGBT cannot be used at high frequencies.